Fever, an elevation in core body temperature above the daily rang for an individual, is a characteristic feature of most infections but is also found in a number of non-infectious diseases such as autoimmune and auto-inflammatory diseases. Definitions of normal body temperature, the pathophysiology of fever, the role of cytokines, and the treatment of fever in adults will be reviewed here. Fever of unknown origin in adults, drug fever, and the treatment of fever in infants and children are discusses separately.